This invention relates to a wobble correction and focusing optical element for a raster optical scanner, and, more particularly, to a wobble correction and focusing optical element with a binary diffractive optic surface and a refractive toroidal surface for the post-polygon mirror optics of a raster optical scanner.
Many conventional raster optical scanners utilize a multi-faceted rotating polygon mirror as the scanning element. A collimated beam of light, as, for example, from a laser, strikes the facets of the rotating polygon which causes the reflected light to revolve about an axis near the center of rotation of the rotating polygon and scan a straight line. This reflected light can be utilized to scan a document as the input of an imaging system or can be used to impinge upon a photosensitive medium, such as a xerographic drum, as the output of an imaging system.
The post-polygon optics of the raster optical scanner are the optical elements in the optical path between the facets of the rotating polygon mirror and the scan line of the raster optical scanner.
State of the art raster optical scanners often use motion compensation optics to reduce or eliminate scan line wobble. Motion compensation optics are also referred to as wobble correction optics and also provide the primary means of focusing the light beam upon the scan line in the cross-scan plane.
Wobble is defined as an error in the post-polygon optics of the optical scanning system caused by the rotating facet not being exactly parallel to the vertical axis. The beam reflected from the facet is thereby angled up or down a small amount resulting in scan line displacement errors in the cross-scan direction.
Angular wobble errors can be caused by several factors. The motor or motor bearings driving the rotating polygon mirror can vibrate during operation. The motor shaft can deviate from its rotational axis. The facets of the polygon mirror can be irregular surfaces which do not lie parallel to each other.
Extreme precision in the manufacture of the motor, bearings, motor shaft and polygon mirror can reduce wobble but not eliminate it. Such precision also increases the steps of production and makes mass production of the scanning system virtually impractical and commercially prohibitive in cost.
Another factor in the high cost of correcting wobble is the high fabrication and assembly tolerances required for the optical scanning system to work efficiently and properly.
The prior art raster optical scanning system 10 of FIG. 1 consists of a pre-polygon mirror optical section 12, a rotating polygon mirror 14 with a plurality of facets and a post-polygon mirror optical section 16 to correct for wobble of the rotating polygon mirror.
A laser diode light source 18 emits a coherent light beam 20 which is collimated in both the scan and cross-scan planes by a multi-element optical collimator 22. The resulting collimated beam 20 passes through a cross-scan cylindrical lens 24. This lens 24 is cylindrical in the cross-scan plane and plano in the scan plane. The lens converges the cross-scan portion of the beam while maintaining the collimation of the scan portion of the beam.
The cross-scan cylindrical lens 24, with the light source 18 and collimator 22, are the elements of the pre-polygon mirror optical section 12.
The beam is converging in the cross-scan plane from the cross-scan cylindrical lens 24 which focuses the beam on a facet 26 of the multi-faceted rotating polygon mirror 14 while the scan plane portion of the beam remains collimated when the beam strikes the facet.
The beam reflected from the facet 26 is still collimated in the scan plane and is now diverging in the cross-scan plane. After reflection from the facet, the beam then passes through an f-theta scan lens 28 consisting of a negative plano-spherical lens 30 and a positive plano-spherical lens 32. This f-theta scan lens configuration converges the beam in the scan plane. The beam then passes through a cross-scan cylindrical lens 34.
This lens 34 is cylindrical and negative in the cross-scan plane, causing an additional divergence of the beam, and plano in the scan plane. The primary focus of the beam in the cross-scan plane upon the scan line is achieved by means of the cylindrical mirror 36 which together with the lens 34 constitutes the cross-scan or motion compensation optics 37. These cross-scan optics 37 in conjunction with the f-theta theta lens 28 will flatten the field curvature of the beam in both the scan and cross-scan planes. Thus, the f-theta scan lens together with the cross-scan cylindrical lens produces a linear scan, flat-field beam focussed upon the image plane of the scan line. The f-theta lens 28 is designed with the cross-scan optics 37 because the cross-scan optics may contribute a small, but non-negligible, amount of distortion, especially at large scan angles.
After passing through the cross-scan cylindrical lens 34, the beam is then reflected off a cylindrical wobble correction mirror 36. This mirror 36 is positive and cylindrical in the cross-scan plane and flat in the scan plane. Thus, the wobble mirror converges the previously diverging cross-scan portion of the beam but allows the converging cross-scan portion of the beam focused by the lens 36 to pass through uneffected.
The reflected beam is focussed onto a photoreceptor or a scan line 38 by the mirror 36 which converges the cross-scan portion of the beam and by the f-theta lens 28 which converges the scan portion of the beam.
The f-theta scan lenses 28 with its negative plano-spherical lens 30 and positive plano-spherical lens 32, together with the cross-scan or motion compensation optics 37 with its cross-scan cylindrical lens 34 and the cylindrical wobble correction mirror 36 are the elements of the post-polygon mirror optical section 16.
The f-theta lens in the scan plane and the motion compensation optics in the cross-scan plane combine to bring the light beam to focus at the scan line in both planes. The scan field of the beam is flattened, the scan is linearized and the wobble is compensated. The small residual focus and position errors which may remain are typically negligible in a well-designed raster optical scanner.
The wobble correction, motion compensation and focusing of the post-polygon mirror optical section 16 of the prior art raster optical scanning system 10 is shown in FIGS. 2 and 3. In the scan plane of FIG. 2, the collimated light beam 20 is reflected off the facet 26 of the rotating polygon mirror 14. The reflected beam 20 is weakly diverged by the negative plano-spherical lens 30 and then converged by the positive plano-spherical lens 32 to focus on the scan line 38. The beam passes through the cross-scan cylindrical lens 34 uneffected and is reflected off the cylindrical wobble correction mirror 36 uneffected.
In the cross-scan plane of FIG. 3, the converging beam 20 is reflected off the facet 26 of the rotating polygon mirror 14. The now diverging beam passes through the plano-spherical lens 30 and the plano-spherical lens 32 uneffected. The diverging beam 20 is weakly further diverged by the cross-scan cylindrical lens 34 and then converged by the cylindrical wobble correction mirror 36 to focus on the scan line 38.
In this optical configuration, the initial optical element serves as a corrector for the subsequent optical element in both planes of the post-polygon mirror optical section of the prior art raster optical scanning system. Thus, as shown in the scan plane of FIG. 2, the initial negative plano-spherical lens 30 is a corrector element which weakly diverges the beam before the subsequent positive plano-spherical lens 32 converges and focuses the beam. As shown in the cross-scan plan of FIG. 3, the initial cross-scan cylindrical lens 34 weakly diverges the beam before the subsequent cylindrical wobble correction mirror 36 converges and focuses the beam.
Currently there are three basic types of of optical elements that are used to perform wobble correction in the post-polygon optics of the optical scanning systems: (1) a wobble correction cylindrical mirror, (2) a toric lens, and (3) a cylindrical lens. Each type has its own unique advantages and disadvantages.
The wobble correction mirror contributes to the correction of cross-scan field curvature but imposes some mechanical constraints on the size of the optical scanning system and how the beam is folded onto the scan plane. The toric lens also has a flat cross-scan field and also allows greater freedom and flexibility in the optical scanning system design, and, if an external fold mirror is used, the critical optical components can be confined to a small space. However, the toric lens is difficult to manufacture and is therefore expensive. The cylinder lens is much easier to produce and also reduces the mechanical packaging constraints, but has a large cross-scan field curvature producing a curved scan line.
All three types of wobble correction elements rely on either reflective or refractive surfaces to perform the desired optical transformations. These surfaces may have shapes that are difficult and expensive to fabricate, and may not have all of the desired optical correction characteristics. For laser diode light sources for the optical scanning system, the waveband of light emitted is small enough so that chromatic correction is typically not required for refractive optics used with laser diode sources.
Another type of surface for an optical element uses the process of diffraction to obtain the desired optical transformation characteristics. These diffractive surfaces have surface profiles that can focus and redirect light, and can be designed to have optical correction properties that are not available with easily manufacturable refractive and reflective surface shapes. In addition, many of these diffractive surface profiles can be fabricated using a multi-level profile structure (binary diffractive optics technology) on a flat substrate. Optical elements that use diffractive surfaces are highly dispersive, more dispersive than refractive elements. In fact, they are so dispersive they cannot be used alone as the primary source of optical power in systems that use laser diodes for optical scanning systems.
Binary diffractive optic lenses are formed by etching or molding very shallow and precise steps or grooves into the surface of a transparent optical element. Binary diffractive optic lenses present substantial cost savings over conventional precision glass or plastic optical lenses. Binary optical elements can be fabricated using the same techniques used to fabricate VLSI circuits, as disclosed in Binary Optics Technology: The Theory and Design of Multi-level Diffractive Optical Elements by G. J. Swanson of the Lincoln Laboratory at the Massachusetts Institute of Technology, (Technical Report 854, 14 August 1989) and the resulting U.S. Pat. No. 4,895,790. A designer develops an idealized diffractire surface structure mathematically, then using a computer, defines a series of precise, microlithographic masks. A mask pattern is contact printed into a photoresist coating using a UV light source and then transferred into the optical substrate by ion milling or plasma etching.
A wobble correction lens for a raster output scanner combines a positive cross-scan plano-cylindrical lens, which provides most of the optical power for focusing the light beam to a scan line, with a diffractive surface, which corrects the cross-scan field curvature of the cross-scan plano-cylindrical lens in recent Xerox U.S. Pat. No. 5,208,701. The diffractive surface will have a multi-level structure (binary diffractive optical surface) which possesses a diffractive phase function that will flatten the cross-scan field curvature of the plano-cylindrical lens.
However, this diffractire cylindrical wobble correction lens is used in conjunction with an additional f-theta scan lens which may contribute significantly to the cross-scan field curvature of the scanning optical system. This wobble correction lens will also effect the scan linearity. Therefore, this wobble correction lens with a binary diffractive surface and a refractive cylindrical surface should be designed and optimized with the additional f-theta scan lens.
It is an object of this invention to provide a post-polygon optics system with fewer and simpler optical elements to provide the functions of the f-theta lens and the cross-scan cylindrical lens and cylindrical wobble correction mirror that will correct wobble and focus an incident light beam to produce a straight scan line.
It is another object of this invention to provide a wobble correction and focusing optical element that combines a refractive toric lens with a binary diffractive optic lens to correct wobble in the scan line and to focus along the scan line.